Guess Number, C++ (gcc)

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/*32 Guess Number
The system has a secret four digits number, in which each digit is in range of one to 6 [1, 6].
Given a four digital number, the system also provide a API that returns the number of correct matched digits.
Design a method to guess this secret number.
*/
// 1) You first can determine what all the digits being used by calling all sequences like '1111', '2222', ..., => save all digits as a set<int> s; 
// 2) Then try to determin each position's potential digits, the return number can be = < >, other than the =, it provide YES, NO answer right away. 
// 3) using a map to save all the calling history.   
// 4) if you want to return the total number guessed, save another private member variable. 
       
/* string target = "1234", 
   guess = "1111" => 1 
   guess = "2222" => 1
   guess = "1234" => 1
   guess = "0000" => 0 
   
   N = 4, M = 6 (1~6), n = N**M = 36*36 ~ 1K 
   1) find which digits are used ?    map<char, int> m; // 6 + 4 + 4 + 4 + 4 = > 22 guess 6(1-6) 
   2) determin eacho position's letter 
   
*/

#include <iostream>
#include <string>
#include <vector>
#include <map>
#include <set>

using namespace std;

class Game{
    string target; 
    int ct; 
    int N; 
public: 
    Game(string tg): target(tg), N((int)tg.size()), ct(0){}
    int guessOne(string guess){
        ct++;
        int res =0; 
        if(guess.size()!=4) return 0;
        for(int i =0; i<guess.size(); i++){
            if(guess[i]==target[i]) 
                res++;
        }
        return res;
    }
    string play(){ 
        
        // try "1111",..."6666"; 
        int res = 0;
        ct = 0; 
        map<string, int> m; // function calling result 
        set<char> s; 
        vector<string> bases = {"1111", "2222", "3333", "4444", "5555","6666"}; 
        for(auto guess: bases){
            res = guessOne(guess); 
            m[guess] = res; 
            if(res==4) return guess;
            if(res>0)
                s.insert(guess[0]); 
        }
        
        // try to determine each position  
        string guess = "1111";
        for(int i=0; i<N; i++) {
            int pre = m[guess];
            for(auto x: s){
                string old = guess;
                guess[i] = x; // new try
                if(m.count(guess))
                    res = m[guess];
                else{
                    res = guessOne(guess);
                    m[guess] = res;
                }
                if(res > pre){ // new
                    break; 
                }
                else if(res < pre) {
                    guess = old; 
                    break;
                }
                else{ // contine with the next dig
                }
            }
        }
          
        return guess; 
    }
    int totalGuess(){
        return ct; 
    }
};

int main() {
    string target = "1456"; 
    Game g(target); 
    string guess = g.play();
    cout << "Number = " << guess << endl;
    cout << "Total try = " << g.totalGuess() << endl; 
    return 0; 
}

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Compilation time: 0.52 sec, absolute running time: 0.07 sec, cpu time: 0.01 sec, memory peak: 3 Mb, absolute service time: 0,6 sec